We are discussing all of the changes, and of course would like to know your opinions, and would appreciate some actual discussion as to why the Pokemon should move. This is how I would rank them, let me know if and why you would change something.
Kingdra: A-
Kingdra is a staple on pretty much all rain teams, it's an extremely powerful mon with great typing, speed in rain, and defenses. Sadly for Kingdra, Ferrothorn is the most used Pokemon in pvp along with Gastrodon and Mantine rising in usage. Kingdra is pretty predictable with its sets (nobody runs hp electric) and easily dealt with by common Pokemon such as Chansey and Tangrowth, along with all the previously mentioned ones. Because its only usage is on rain (besides the extremely rare Critdra set), along with lack of versatility, I do not believe it belongs in the same tier as other A-rank Pokemon.
Weavile: A+
With the introduction of Pursuit, this already extremely dangerous Pokemon became even better. It is the fastest commonly seen Pokemon with access to priority and coverage moves to beat some counters. With Pursuit, it is easily the best revenge killer in the meta and is one of the most splashable Pokemon, being seen on all sorts of teams from stall to hyper offense. With a Choice Band on, Weavile has a chance to OHKO Latios after rocks damage, with Pursuit, even if it stays in. The few downsides that made me hesitant to put it in A+ are its lack of set versatility and its weakness to Stealth Rock.
Hippowdon: B+
Although it's a decent weather setter and stealth rocker, I find very little reason to ever use this Pokemon on a team. Tyranitar is usually the better option on sand teams and there are many better defensive options to handle meta Pokemon. Many of the Pokemon it is supposed to handle are uncommon in the PRO meta or not even in the game. Some examples are electric types like Thundurus, Mega Manectric, Zapdos, Raikou, and some fire types like Talonflame, Mega Charizard X and Y.
Klefki: B+
It's a good dual screens user and spikes setter for hyper offense, but besides that, it doesn't have many other uses. As a defensive Pokemon, it loses to almost all of the top tier Pokemon. Klefki's main faults are a lack of bulk and reliable recovery as well as losing to many common hazard removers such as Rotom-Wash, Excadrill, and Gliscor.
Starmie: B+
I'll start by saying I really like Starmie, so maybe I'm a bit biased. I had a lot of success with it in Gen 6 OU on Showdown. It is a good offensive spinner for hyper offensive teams and a reliable spinner on bulky offense or balance. It recently gained its hidden ability, Analytic, which helps the offensive set that lacked power without it. Sadly, at the same time, Pursuit got added into the game which Starmie has no way of playing around. The main problem is that Starmie is still missing a necessary move for the defensive set: Reflect Type. This would allow Starmie to predict being Pursuit trapped and would give it a fighting chance in the match-up. Without it, a Pursuit trapper could simply switch in on Starmie and remove it from the match. This, of course, only concerns the bulky set, as even a Choice Scarf Tyranitar wouldn't want to switch into a Life Orb, Analytic-boosted Hydro Pump. Once Reflect Type is coded, I will consider rising Starmie up to A- once again.
Arcanine: B
I agree with David, it's just completely outclassed offensively and defensively by better Pokemon even in the same tier.
Feraligatr: B+
I'm happy with where it is. It's a decent set up sweeper with Dragon Dance and a decent wallbreaker with Swords Dance, along with access to Sheer Force, priority, and good coverage.
Gastrodon: A-
With all of the recent changes to the meta, Gastrodon has become a very solid defensive Pokemon. The biggest thing it has going for it is being one of the best answers to the ban-worthy Greninja lacking grass coverage. Along with Greninja, it also walls other very common top tier Pokemon such as Heatran lacking grass coverage, most Rotom variants, and most rain Pokemon. Unlike Mantine, it can even handle Kabutops and mixed Dragonite.
Mamoswine: B+ (not sure)
This one is very tricky for me. It seems a bit better than some Pokemon in B+, but worse than some in A-. It has a good offensive typing along with dark and fighting coverage, but is walled by some common defensive Pokemon such as Rotom-Wash and Skarmory. It has a lot of downsides such as frail defenses with an ice typing, slower than many other offensive Pokemon, and an ability that doesn't help it offensively. One plus side is having access to Stealth Rock and Oblivious, so it could be used as a decent suicide lead.
Mantine: A-
I think this Pokemon is extremely underrated right now, being able to handle some extremely scary threats such as most Greninja sets, Volcarona, Choice Scarf Excadrill, Gengar, Alakazam, Nidoking lacking Thunderbolt, Gyarados, and Hydreigon. It fills a very similar role to Gastrodon, the main differences being Mantine having access to Defog and dealing with different threats.
Omastar: B
This is another Pokemon I love to use in rain. It is the hardest hitting rain sweeper. Its Hydro Pump can 2HKO Blissey after rocks in rain and can even run a set with Knock Off to beat Chansey. If you look at some damage calculations, you'll be wondering why you haven't used this thing sooner. Sadly, with the rise of Mantine and Gastrodon, as well as the nerf to Talonflame, Omastar is harder to use than ever before.
Jellicent: B-
This used to be a pretty reliable spin blocker, but with the addition of Pursuit and many new Pokemon (mainly Greninja and Rotom-Wash), there really isn't any place for it anymore. It does have access to reliable recovery and Will-O-Wisp, but its bulk is just too low to be able to switch into neutral attacks from things like Life Orb Excadrill and Life Orb Garchomp. However, right now, in Gen 6 OU, there is a rise in usage of Colbur Berry Jellicent to better take on dark types. This could be something to consider in PRO with the addition of berries, but the downside to this is losing Leftovers recovery.
Ninetales: B-
Smeargle: B
Scarf Spore OP, that's all.
Shuckle: B-
Talonflame: B
Torkoal: B-